The quality of a silicon substrate used for manufacturing semiconductor devices greatly influences the yield and reliability of semiconductor devices. In view of this, various standards are prescribed to maintain the quality of a silicon substrate. Specifically, there are prescribed standards for resistivity, resistivity variation, interstitial oxygen concentration, oxidation-induced stacking fault (OSF), crystal orientation, size, planar degree, bowing, external appearance (nick, scratch, spike, haze, stain, etc.), and the like. Presently, there is only an OSF standard used as a standard for the crystal property of a region (device active region) near at the substrate surface on which a device is formed.
The integration density of a semiconductor device is becoming higher year after year. It is therefore difficult to maintain the yield and reliability of semiconductor devices by using only the above-described conventional standards. Particularly, in the case of the outer appearance standard for inspecting the substrate surface on which a device is formed, an expert person visually inspects using oblique rays. Such a conventional method provides an ability of detecting particles and unevenness only about 0.3 .mu.m. Therefore, if a semiconductor substrate having passed such an inspection is used for manufacturing semiconductor devices by a design rule 0.8 .mu.m or smaller, serious problems may often occur with respect to the yield and reliability.
Furthermore, as described above, in order to maintain the yield and reliability of semiconductor integrated circuit elements, it is necessary to make an Si substrate surface area or element active region completely non-defective. In addition, in order to deal with metallic contamination during the element manufacturing processes, it has become necessary to form crystal defects within a Si substrate under excellent controllability.
More in detail, a semiconductor device is formed on a silicon wafer surface by using various processes such as thermal treatment, etching, and film deposition. It is desirable that the structure of a wafer is preferably has no defect near at the wafer surface or element active region, and has an intrinsic gettering (IG) structure having bulk micro defects (BMD) for gettering metallic impurities. Therefore, the IG structure of a ULSI substrate is often formed by an in-process IG, i.e., by thermal treatments during processes including a high temperature process (up to 1200.degree. C.), low temperature process (600.degree. to 800.degree. C.), and middle temperature process (up to 1000.degree. C.). With this IG process, however, oxygen precipitation of low concentration is often present even in the non-defective layer at the Si substrate surface. Therefore, the electric characteristics are degraded by P-N junction leakage or the like of a semiconductor integrated circuit element.
The above-described visual inspection can detect only about 0.3 .mu.m particles and surface defects on a substrate surface. If circuit elements are manufactured using a substrate having passed such a visual inspection, there has often occurred a problem of yield and reliability. It has been found by the present inventors that a fine defect of 0.1 .mu.m or larger on a substrate surface takes an important roll in lowering the yield and the like.
As shown in FIG. 4, elements formed on a substrate having a fine defect density of 0.5 point/cm.sup.2 or lower detected with a surface defect detecting apparatus showed a high yield.
However, the fine defect density of the surface of a generally used Si substrate is mostly 1/cm.sup.2 or higher. It is difficult to obtain a high quality substrate having the fine defect density of 0.5 point/cm.sup.2 or lower.
An epitaxial wafer is used in some cases to obtain a perfectly non-defective layer.
A description will be given below for a conventional method of evaluating an element active region as to whether or not it is really non-defective or not. First, a wafer is cleaved. A selective etching is carried out using a Wrigt etching method or a Secco etching method to expose BMDs which are then observed with a microscope. With this method, however, the size of observable defects has a limit. A BMD is considered that a nucleus originally present in a crystal grows by precipitation of oxygen around the nucleus during a thermal process. It is not clear that to what degree a BMD has grown can be observed by the above-described method. However, it is not possible to observe, for example, a nucleus itself or a BMD grown only to a small degree.
Recently, an infrared tomography method has been adopted to evaluate the distribution and number of BMDs by radiating an infrared ray (wavelength 1.06 .mu.m or 1.32 .mu.m) and measuring the infrared ray scattered by BMDs. However, an image is sometimes disturbed depending upon the configuration of a wafer surface. It is therefore impossible to measure small size BMDs.
As described above, it has been impossible heretofore to properly evaluate a semiconductor substrate. In addition, it is difficult to obtain a semiconductor substrate suitable for use in manufacturing highly integrated semiconductor devices, so that it is also difficult to obtain high quality semiconductor devices.